1. Field of the Invention
The invention relates to a method of forming an X-ray layer image of an object, and to an X-ray device for carrying out such a method.
2. Description of the Related Art
The formation of X-ray layer images of an object by utilizing tomosynthesis has been known for a long time. In tomosynthesis, the X-ray source and the X-ray detector are displaced in opposite directions in planes extending parallel to the object being examined and parallel to one another; and then X-ray projection images of the object being examined are acquired from different positions. Using suitable reconstruction methods, layer images of layers of the object being examined can be formed from such X-ray projection images, where said layer images extend parallel to the planes in which the X-ray source and the X-ray detector were moved.
It is also known to move the X-ray source and the X-ray detector along circular trajectories around the object being examined, for example, by means of a C-arm. In the prior art, it has been assumed that a complete data set is necessary for the reconstruction of high quality layer images; for this purpose the X-ray source and the X-ray detector must be displaced through an angular range of at least 180xc2x0 around the object being examined. This 180xc2x0 displacement was also necessary because layer images were to be formed not only in a single plane or in parallel planes through the object being examined, but also in a plurality of preferably mutually perpendicular planes through the object being examined.
It is an object of the present invention to provide a method of and a device for forming an X-ray layer image of adequate image quality in a single plane or in mutually parallel planes while using less images, less angular range, or a shorter period of time.
The invention is based on the idea that the formation of a layer image in a single plane or in parallel planes does not necessitate displacement of the X-ray source and the X-ray detector through at least 180xc2x0 around the object being examined; in other words, that acquisition of X-ray projection images from an angular range of less than 180xc2x0 suffices to form an X-ray layer image of adequate image quality. It has been found that artefacts occur only to a limited extent in an angular range of less than 180xc2x0; such artefacts can be ignored for clinical applications. This is particularly true for an X-ray layer image that lies in a plane perpendicular to the bisector of the angular range in which the X-ray source and the X-ray detector are displaced. When use is made of a C-arm X-ray device, as in a preferred embodiment, the bisector thus corresponds to the central pivot position of the X-ray source when it is pivoted from side to side. In other words, the bisector is the center line of the overall angle covering the angular range.
Moreover, the X-ray layer images in accordance with the invention are calculated directly from the X-ray projection images, as opposed to prior art methods where they are calculated from a 3D data set determined from the X-ray projection images.
Thus, in accordance with the invention, X-ray layer images can be formed in parallel layers in a simple and fast manner, because the angular displacement range to be covered is smaller than in the prior art methods. On the other hand, the speed of rotation of the X-ray source and the X-ray detector may also be reduced and the period of time during which a contrast agent is present in the object being examined can be used more effectively.
In a further version of the invention, the position of the angular range relative to the object being examined can be changed so as to image differently oriented layers.
In preferred embodiments of the method in accordance with the invention, the total angular range is from 90xc2x0 to 180xc2x0 or, depending on the relevant application, to even less than 90xc2x0. It has been found that an image quality which is sufficient for various clinical applications as well as for adequate suppression of artefacts can also be achieved using an angular range of less than 150xc2x0. However, when the angular range is reduced further, contour lines of object details in the examination zone are become more and more blurred.
In order to further reduce the time required for the acquisition of the X-ray projection images and the formation of the X-ray layer image, the number of X-ray projection images to be acquired for the formation of the X-ray layer image can be limited in further preferred embodiments of the invention. Granted, the image quality generally is higher when the number of projection images is larger, because the reconstruction artefacts are more spread. However, it has been found that adequate image quality can be achieved by using a number of no more than 100 X-ray projection images; and, for specific applications, a maximum number of 80 X-ray projection images is sufficient. The reduction of the number of images also leads to a reduction of the radiation dose.
Furthermore, in a preferred embodiment, a plurality of essentially parallel X-ray layer images of the object being examined are formed from the acquired X-ray projection images. This is possible because, in accordance with the invention, the image quality suffices for essentially parallel X-ray layer images, in contrast to the image quality of X-ray layer images of layers that are situated essentially parallel to the bisector would be significantly poorer and inadequate for clinical applications.
In accordance with the invention, a C-arm X-ray device is advantageously used for the acquisition of the x-ray projection images.
It may also be advantageous to combine a plurality of X-ray layer images of neighboring thin layers so as to form an X-ray layer image of a thicker layer.
For the acquisition of the X-ray projection images, the X-ray source and the X-ray detector can be displaced either along a circular trajectory or in opposite parallel planes around the object being examined as in the case of the known tomosynthesis. Furthermore, it may also be arranged that only the X-ray source or only the X-ray detector is displaced in a single plane around the object being examined while the other element is stationary.